Photoplethysmography (PPG) refers to acquiring a volumetric organ measurement by optical means. Frequently, pulse oximeters are employed, which detect changes in light absorption properties of the human skin. Typically, a transmissive or reflective blood PPG sensor monitors the perfusion of blood to the dermis and subcutaneous tissue of the skin through absorption measurement at a specific wavelength. Besides light originating from blood, there is a far greater portion of light detected, which originates from tissue and ambient light.
Photoplethysmography signals comprise a very small AC signal (the actual plethysmogram) on top of a very large (unwanted) DC offset signal. The DC offset signal comprises signals originating from skin and tissue, and from a considerable part of ambient light. Unfortunately, the amount of ambient light detected is not constant, but varies due to changing ambient light conditions and due to motion artifacts (caused, e.g., by the coupling between the photoplethysmography sensor and the skin). The temporal rate of change of detected ambient light includes frequencies in the photoplethysmography frequency band of interest. This prohibits simple frequency domain filtering, because filtering out these frequencies (in the hope of suppressing the detected ambient light) would also filter (or significantly suppress) frequencies of the photoplethysmography frequency band of interest.
Currently known mechanisms for ambient light rejection include for example DC-restore circuits, which sample the ambient light periodically when the photoplethysmography excitation light (such as, e.g., a light-emitting diode, LED) is temporarily turned off. In a different time slot (e.g., when the LED is turned on) a sample is taken which contains both ambient and the photoplethysmography signal. By subtracting the signal with the photoplethysmography excitation light turned off from the signal with the photoplethysmography excitation light turned on, an “offset-corrected” photoplethysmography signal is obtained, which does not exhibit interference from ambient light. Typically, this sampling is done after a transimpedance amplifier (TIA) has converted and amplified the photocurrent generated by the detector into a voltage. Alternatively and/or additionally, this sampling is done completely in the digital domain after the signal has been processed by an analog-to-digital converter (ADC).
A number of problems and disadvantages are present in conventional photoplethysmography sensors. First, the amount of ambient light detected can be considerable. This means that when designing the amplifier, a certain amount of the dynamic range available must be reserved for properly detecting the ambient light, resulting in a sub-optimal amplifier design.
Additionally, if the subtraction of the ambient signal is done in the digital domain (i.e., after analog-to-digital conversion), then a number of ADC bits have to be reserved for the ambient light. Reserving ADC bits for the ambient light however limits the resolution available for photoplethysmography signals.
However, if the subtraction is done directly after processing by the TIA, a sample-and-hold circuit is needed to hold the ambient value (i.e., the measurement value corresponding to the ambient light) until the next sampling period of the photoplethysmography signal. The gain accuracy of this sample-and-hold signal determines the efficacy of the compensation.
One option to address the problems of conventional photoplethysmography sensors is to employ a factory calibration step of the sample-and-hold element. Such an additional factory calibration step however adds manufacturing costs and is thus less preferable.
U.S. Pat. No. 7,740,591 discloses a plethysmography sensor. This sensor comprises an ambient light canceling circuit that receives the output of transimpedance differential amplifiers. The ambient light canceling circuit operates as follows: when timing control circuit has both the Red and IR LED's off, the ambient light is the only light the sensor has for an output. The Ambient light is sampled, and the value of the signal is held in a capacitor tied to ground using a FET. When the FET is turned off, the value stored in the capacitor is used in the path of the Red and IR signal string. This stored value in the capacitor removes the error of the ambient light.
U.S. Pat. No. 6,381,479B1 discloses a system for providing an improved DC and low frequency signal rejection in a photoplethysmographic measurement instrument. The system is used in a measurement instrument which includes at least two signal sources for transmitting light signals at least at two wavelengths through a tissue of a test subject and a detector for converting light signals transmitted through the tissue into a detector output signal. The system includes a DC restoration which removes DC and low frequency signal components from the detector output signal prior to amplification thereof so as to avoid saturating amplified output signal with the low frequency signal component. The DC restoration is configured to continuously remove low frequency signal component from the detector signal during dark intervals when the signal sources are deactivated, as well as during light intervals when one of the signal sources is activated. In one embodiment, the DC restoration is embodied in the form of a DC restoration circuit which comprises a transimpedance amplifier which receives the detector output signal and produces an amplifier output signal and an integrator feedback loop which receives the amplified output signal and produces a bias current, wherein the bias current is used to subtract DC and low frequency signal components from the detector output signal prior to amplification of the detector signal by the amplifier.